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The Chern number is often used to distinguish different topological phases of matter in two-dimensional electron systems. A fast and efficient coupling-matrix method is designed to calculate the Chern number in finite crystalline and disordered systems. To show its effectiveness, we apply the approach to the Haldane model and the lattice Hofstadter model, and obtain the correct quantized Chern numbers. The disorder-induced topological phase transition is well reproduced, when the disorder strength is increased beyond the critical value. We expect the method to be widely applicable to the study of topological quantum numbers. [ABSTRACT FROM AUTHOR]

Measurements of three-junction flux qubits, both single flux qubits and coupled flux qubits, using a coupled direct current superconducting quantum interference device (dc-SQUID) for readout are reported. The measurement procedure is described in detail. We performed spectroscopy measurements and coherent manipulations of the qubit states on a single flux qubit, demonstrating quantum energy levels and Rabi oscillations, with Rabi oscillation decay time TRabi = 78 ns and energy relaxation time T1 = 315 ns. We found that the value of TRabi depends strongly on the mutual inductance between the qubit and the magnetic coil. We also performed spectroscopy measurements on inductively coupled flux qubits. [ABSTRACT FROM AUTHOR]

Epitaxial La2/3Ca1/3MnO3 thin films grown on LaAlO3 (001) substrates were irradiated with low-energy 120-keV H+2 ions over doses ranging from 1012 ions/cm2 to 1017 ions/cm2. The irradiation suppresses the intrinsic insulator—metal (I—M) transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 × 1015 ions/cm2 is in a threshold state of the electric insulator where the I—M transition is absent. In an external field of 4 T or higher, the I—M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field. [ABSTRACT FROM AUTHOR]

Besides serving as promising candidates for realizing quantum computing, superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested, giving rise to a vast field of intensive research work both theoretically and experimentally. In this paper we report our work on the fabrication of superconducting quantum circuits, starting from its building blocks: Al/AlOx/Al Josephson junctions. By using electron beam lithography patterning and shadow evaporation, we have fabricated aluminum Josephson junctions with a controllable critical current density (jc) and wide range of junction sizes from 0.01 µm² up to 1 µm². We have carried out systematical studies on the oxidation process in fabricating Al/AlOx/Al Josephson junctions suitable for super-conducting flux qubits. Furthermore, we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits. [ABSTRACT FROM AUTHOR]

Realization of a flexible and tunable coupling scheme among qubits is critical for scalable quantum information processing. Here, we design and characterize a tunable coupling element based on Josephson junction, which can be adapted to an all-to-all connected circuit architecture where multiple Xmon qubits couple to a common coplanar waveguide resonator. The coupling strength is experimentally verified to be adjustable from 0 MHz to about 40 MHz, and the qubit lifetime can still be up to in the presence of the coupling element. [ABSTRACT FROM AUTHOR]

Superconducting quantum bits (qubits) and circuits are the leading candidate for the implementation of solid-state quantum computation. They have also been widely used in a variety of studies of quantum physics, atomic physics, quantum optics, and quantum simulation. In this article, we will present an overview of the basic principles of the superconducting qubits, including the phase, flux, charge, and transmon (Xmon) qubits, and the progress achieved so far concerning the improvements of the device design and quantum coherence property. Experimental studies in various research fields using the superconducting qubits and circuits will be briefly reviewed. [ABSTRACT FROM AUTHOR]

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device (SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as are fabricated for the qubit, in which the number of the decoherence-causing two-level systems (TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T1 and dephasing time on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance. [ABSTRACT FROM AUTHOR]

Hybrid quantum system of negatively charged nitrogen−vacancy (NV−) centers in diamond and superconducting qubits provide the possibility to extend the performances of both systems. In this work, we numerically simulate the coupling strength between NV− ensembles and superconducting flux qubits and obtain a lower bound of 1016 cm−3 for NV− concentration to achieve a sufficiently strong coupling of 10 MHz when the gap between NV-ensemble and flux qubit is 0. Moreover, we create NV− ensembles in different types of diamonds by 14N+ and 12C+ ion implantation, electron irradiation, and high temperature annealing. We obtain an NV− concentration of 1.05 × 1016 cm−3 in the diamond with 1-ppm nitrogen impurity, which is expected to have a long coherence time for the low nitrogen impurity concentration. This shows a step toward performance improvement of flux qubit-NV− hybrid system. [ABSTRACT FROM AUTHOR]

Superconducting nanowire single photon detector (SNSPD), as a new type of superconducting single photon detector (SPD), has a broad application prospect in quantum communication and other fields. In order to prepare SNSPD with high performance, it is necessary to fabricate a large area of uniform meander nanowires, which is the core of the SNSPD. In this paper, we demonstrate a process of patterning ultra-thin NbN films into meander-type nanowires by using the nano-imprint technology. In this process, a combination of hot embossing nano-imprint lithography (HE-NIL) and ultraviolet nano-imprint lithography (UV-NIL) is used to transfer the meander nanowire structure from the NIL Si hard mold to the NbN film. We have successfully obtained a NbN nanowire device with uniform line width. The critical temperature (Tc) of the superconducting NbN meander nanowires is about 5 K and the critical current (Ic) is about 3.5 μA at 2.5 K. [ABSTRACT FROM AUTHOR]

Micro-patterning is considered to be a promising way to analyze phase-separated manganites. We investigate resistance in micro-patterned La0.325Pr0.3Ca0.375MnO3 wires with width of 10 μm, which is comparable to the phase separation scale in this material. A reentrant of insulating state at the metal—insulator temperature Tp is observed and a giant resistance change of over 90% driven by electric field is achieved by suppression of this insulating state. This resistance change is mostly reversible. The I—V characteristics are measured in order to analyze the origin of the giant electroresistance and two possible explanations are proposed. [ABSTRACT FROM AUTHOR]

Electric double-layer field effect experiments were performed on ultrathin films of La0.325Pr0.3Ca0.375MnO3, which is noted for its micrometer-scale phase separation. A clear change of resistance up to 220% was observed and the characteristic metal—insulator transition temperature TP was also shifted. The changes of both the resistance and TP suggest that the electric field induced not only tuning of the carrier density but also rebalancing of the phase separation states. The change of the charge-ordered insulating phase fraction was estimated to be temperature dependent, and a maximum of 16% was achieved in the phase separation regime. This tuning effect was partially irreversible, which might be due to an oxygen vacancy migration that is driven by the huge applied electric field. [ABSTRACT FROM AUTHOR]

We report the fabrication and the study of superconducting properties of ultra-thin Nb superconducting meander nanowires, which can be used as superconducting nanowire single-photon detectors (SNSPDs). The ultra-thin (about 7-nm thick) Nb films are patterned into micro-bridges, and 100-nm wide meander nanowires by using e-beam lithography (EBL). The average transition temperature (Tc) of the nanowires is about 4.8 K and the critical current density jc is about 2.8 × 106 A/cm2. Superconducting characteristics of the specimens at different applied magnetic fields up to 8 T (parallel or perpendicular to the specimen) are systematically investigated. The normalized temperature t (= T/Tc) dependences of the parallel critical field (Hc‖) for both the micro-bridge and the meander nanowire are almost the same, following the Ginzburg and Landau (GL) formalism for ultra-thin films. However, in perpendicular field and in the vicinity of Tc (> 0.95Tc), the critical field Hc‖ of the nanowire exhibits a down-turn curvature nonlinear temperature dependence while the micro-bridge displays a linear temperature dependence. The nonlinear behavior of Hc⊥ in the nanowire is believed to be due to the fact that in the vicinity of Tc the coherence length becomes larger than the line width. Additionally, the localization of carriers in the nanowire could also contribute to the nonlinear behavior. The resistive transitions could be described by the phase-slip model for quasi-one-dimensional system. Moreover, the hysteresis in I–V curve of the meander nanowires can be illustrated by a simple model of localized normal hotspot maintained by Joule heating. [ABSTRACT FROM AUTHOR]